1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use core::cmp;

use crate::tables::grapheme::GraphemeCat;

/// External iterator for grapheme clusters and byte offsets.
///
/// This struct is created by the [`grapheme_indices`] method on the [`UnicodeSegmentation`]
/// trait. See its documentation for more.
///
/// [`grapheme_indices`]: trait.UnicodeSegmentation.html#tymethod.grapheme_indices
/// [`UnicodeSegmentation`]: trait.UnicodeSegmentation.html
#[derive(Clone)]
pub struct GraphemeIndices<'a> {
    start_offset: usize,
    iter: Graphemes<'a>,
}

impl<'a> GraphemeIndices<'a> {
    #[inline]
    /// View the underlying data (the part yet to be iterated) as a slice of the original string.
    ///
    /// ```rust
    /// # use unicode_segmentation::UnicodeSegmentation;
    /// let mut iter = "abc".grapheme_indices(true);
    /// assert_eq!(iter.as_str(), "abc");
    /// iter.next();
    /// assert_eq!(iter.as_str(), "bc");
    /// iter.next();
    /// iter.next();
    /// assert_eq!(iter.as_str(), "");
    /// ```
    pub fn as_str(&self) -> &'a str {
        self.iter.as_str()
    }
}

impl<'a> Iterator for GraphemeIndices<'a> {
    type Item = (usize, &'a str);

    #[inline]
    fn next(&mut self) -> Option<(usize, &'a str)> {
        self.iter
            .next()
            .map(|s| (s.as_ptr() as usize - self.start_offset, s))
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}

impl<'a> DoubleEndedIterator for GraphemeIndices<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<(usize, &'a str)> {
        self.iter
            .next_back()
            .map(|s| (s.as_ptr() as usize - self.start_offset, s))
    }
}

/// External iterator for a string's
/// [grapheme clusters](http://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries).
///
/// This struct is created by the [`graphemes`] method on the [`UnicodeSegmentation`] trait. See its
/// documentation for more.
///
/// [`graphemes`]: trait.UnicodeSegmentation.html#tymethod.graphemes
/// [`UnicodeSegmentation`]: trait.UnicodeSegmentation.html
#[derive(Clone, Debug)]
pub struct Graphemes<'a> {
    string: &'a str,
    cursor: GraphemeCursor,
    cursor_back: GraphemeCursor,
}

impl<'a> Graphemes<'a> {
    #[inline]
    /// View the underlying data (the part yet to be iterated) as a slice of the original string.
    ///
    /// ```rust
    /// # use unicode_segmentation::UnicodeSegmentation;
    /// let mut iter = "abc".graphemes(true);
    /// assert_eq!(iter.as_str(), "abc");
    /// iter.next();
    /// assert_eq!(iter.as_str(), "bc");
    /// iter.next();
    /// iter.next();
    /// assert_eq!(iter.as_str(), "");
    /// ```
    pub fn as_str(&self) -> &'a str {
        &self.string[self.cursor.cur_cursor()..self.cursor_back.cur_cursor()]
    }
}

impl<'a> Iterator for Graphemes<'a> {
    type Item = &'a str;

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let slen = self.cursor_back.cur_cursor() - self.cursor.cur_cursor();
        (cmp::min(slen, 1), Some(slen))
    }

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        let start = self.cursor.cur_cursor();
        if start == self.cursor_back.cur_cursor() {
            return None;
        }
        let next = self.cursor.next_boundary(self.string, 0).unwrap().unwrap();
        Some(&self.string[start..next])
    }
}

impl<'a> DoubleEndedIterator for Graphemes<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        let end = self.cursor_back.cur_cursor();
        if end == self.cursor.cur_cursor() {
            return None;
        }
        let prev = self
            .cursor_back
            .prev_boundary(self.string, 0)
            .unwrap()
            .unwrap();
        Some(&self.string[prev..end])
    }
}

#[inline]
pub fn new_graphemes<'b>(s: &'b str, is_extended: bool) -> Graphemes<'b> {
    let len = s.len();
    Graphemes {
        string: s,
        cursor: GraphemeCursor::new(0, len, is_extended),
        cursor_back: GraphemeCursor::new(len, len, is_extended),
    }
}

#[inline]
pub fn new_grapheme_indices<'b>(s: &'b str, is_extended: bool) -> GraphemeIndices<'b> {
    GraphemeIndices {
        start_offset: s.as_ptr() as usize,
        iter: new_graphemes(s, is_extended),
    }
}

// maybe unify with PairResult?
// An enum describing information about a potential boundary.
#[derive(PartialEq, Eq, Clone, Debug)]
enum GraphemeState {
    // No information is known.
    Unknown,
    // It is known to not be a boundary.
    NotBreak,
    // It is known to be a boundary.
    Break,
    // The codepoint after is a Regional Indicator Symbol, so a boundary iff
    // it is preceded by an even number of RIS codepoints. (GB12, GB13)
    Regional,
    // The codepoint after is Extended_Pictographic,
    // so whether it's a boundary depends on pre-context according to GB11.
    Emoji,
}

/// Cursor-based segmenter for grapheme clusters.
///
/// This allows working with ropes and other datastructures where the string is not contiguous or
/// fully known at initialization time.
#[derive(Clone, Debug)]
pub struct GraphemeCursor {
    // Current cursor position.
    offset: usize,
    // Total length of the string.
    len: usize,
    // A config flag indicating whether this cursor computes legacy or extended
    // grapheme cluster boundaries (enables GB9a and GB9b if set).
    is_extended: bool,
    // Information about the potential boundary at `offset`
    state: GraphemeState,
    // Category of codepoint immediately preceding cursor, if known.
    cat_before: Option<GraphemeCat>,
    // Category of codepoint immediately after cursor, if known.
    cat_after: Option<GraphemeCat>,
    // If set, at least one more codepoint immediately preceding this offset
    // is needed to resolve whether there's a boundary at `offset`.
    pre_context_offset: Option<usize>,
    // The number of RIS codepoints preceding `offset`. If `pre_context_offset`
    // is set, then counts the number of RIS between that and `offset`, otherwise
    // is an accurate count relative to the string.
    ris_count: Option<usize>,
    // Set if a call to `prev_boundary` or `next_boundary` was suspended due
    // to needing more input.
    resuming: bool,
    // Cached grapheme category and associated scalar value range.
    grapheme_cat_cache: (u32, u32, GraphemeCat),
}

/// An error return indicating that not enough content was available in the
/// provided chunk to satisfy the query, and that more content must be provided.
#[derive(PartialEq, Eq, Debug)]
pub enum GraphemeIncomplete {
    /// More pre-context is needed. The caller should call `provide_context`
    /// with a chunk ending at the offset given, then retry the query. This
    /// will only be returned if the `chunk_start` parameter is nonzero.
    PreContext(usize),

    /// When requesting `prev_boundary`, the cursor is moving past the beginning
    /// of the current chunk, so the chunk before that is requested. This will
    /// only be returned if the `chunk_start` parameter is nonzero.
    PrevChunk,

    /// When requesting `next_boundary`, the cursor is moving past the end of the
    /// current chunk, so the chunk after that is requested. This will only be
    /// returned if the chunk ends before the `len` parameter provided on
    /// creation of the cursor.
    NextChunk, // requesting chunk following the one given

    /// An error returned when the chunk given does not contain the cursor position.
    InvalidOffset,
}

// An enum describing the result from lookup of a pair of categories.
#[derive(PartialEq, Eq)]
enum PairResult {
    NotBreak, // definitely not a break
    Break,    // definitely a break
    Extended, // a break iff not in extended mode
    Regional, // a break if preceded by an even number of RIS
    Emoji,    // a break if preceded by emoji base and (Extend)*
}

#[inline]
fn check_pair(before: GraphemeCat, after: GraphemeCat) -> PairResult {
    use self::PairResult::*;
    use crate::tables::grapheme::GraphemeCat::*;
    match (before, after) {
        (GC_CR, GC_LF) => NotBreak,                                 // GB3
        (GC_Control, _) => Break,                                   // GB4
        (GC_CR, _) => Break,                                        // GB4
        (GC_LF, _) => Break,                                        // GB4
        (_, GC_Control) => Break,                                   // GB5
        (_, GC_CR) => Break,                                        // GB5
        (_, GC_LF) => Break,                                        // GB5
        (GC_L, GC_L) => NotBreak,                                   // GB6
        (GC_L, GC_V) => NotBreak,                                   // GB6
        (GC_L, GC_LV) => NotBreak,                                  // GB6
        (GC_L, GC_LVT) => NotBreak,                                 // GB6
        (GC_LV, GC_V) => NotBreak,                                  // GB7
        (GC_LV, GC_T) => NotBreak,                                  // GB7
        (GC_V, GC_V) => NotBreak,                                   // GB7
        (GC_V, GC_T) => NotBreak,                                   // GB7
        (GC_LVT, GC_T) => NotBreak,                                 // GB8
        (GC_T, GC_T) => NotBreak,                                   // GB8
        (_, GC_Extend) => NotBreak,                                 // GB9
        (_, GC_ZWJ) => NotBreak,                                    // GB9
        (_, GC_SpacingMark) => Extended,                            // GB9a
        (GC_Prepend, _) => Extended,                                // GB9b
        (GC_ZWJ, GC_Extended_Pictographic) => Emoji,                // GB11
        (GC_Regional_Indicator, GC_Regional_Indicator) => Regional, // GB12, GB13
        (_, _) => Break,                                            // GB999
    }
}

impl GraphemeCursor {
    /// Create a new cursor. The string and initial offset are given at creation
    /// time, but the contents of the string are not. The `is_extended` parameter
    /// controls whether extended grapheme clusters are selected.
    ///
    /// The `offset` parameter must be on a codepoint boundary.
    ///
    /// ```rust
    /// # use unicode_segmentation::GraphemeCursor;
    /// let s = "हिन्दी";
    /// let mut legacy = GraphemeCursor::new(0, s.len(), false);
    /// assert_eq!(legacy.next_boundary(s, 0), Ok(Some("ह".len())));
    /// let mut extended = GraphemeCursor::new(0, s.len(), true);
    /// assert_eq!(extended.next_boundary(s, 0), Ok(Some("हि".len())));
    /// ```
    pub fn new(offset: usize, len: usize, is_extended: bool) -> GraphemeCursor {
        let state = if offset == 0 || offset == len {
            GraphemeState::Break
        } else {
            GraphemeState::Unknown
        };
        GraphemeCursor {
            offset: offset,
            len: len,
            state: state,
            is_extended: is_extended,
            cat_before: None,
            cat_after: None,
            pre_context_offset: None,
            ris_count: None,
            resuming: false,
            grapheme_cat_cache: (0, 0, GraphemeCat::GC_Control),
        }
    }

    fn grapheme_category(&mut self, ch: char) -> GraphemeCat {
        use crate::tables::grapheme as gr;
        use crate::tables::grapheme::GraphemeCat::*;

        if ch <= '\u{7e}' {
            // Special-case optimization for ascii, except U+007F.  This
            // improves performance even for many primarily non-ascii texts,
            // due to use of punctuation and white space characters from the
            // ascii range.
            if ch >= '\u{20}' {
                GC_Any
            } else if ch == '\n' {
                GC_LF
            } else if ch == '\r' {
                GC_CR
            } else {
                GC_Control
            }
        } else {
            // If this char isn't within the cached range, update the cache to the
            // range that includes it.
            if (ch as u32) < self.grapheme_cat_cache.0 || (ch as u32) > self.grapheme_cat_cache.1 {
                self.grapheme_cat_cache = gr::grapheme_category(ch);
            }
            self.grapheme_cat_cache.2
        }
    }

    // Not sure I'm gonna keep this, the advantage over new() seems thin.

    /// Set the cursor to a new location in the same string.
    ///
    /// ```rust
    /// # use unicode_segmentation::GraphemeCursor;
    /// let s = "abcd";
    /// let mut cursor = GraphemeCursor::new(0, s.len(), false);
    /// assert_eq!(cursor.cur_cursor(), 0);
    /// cursor.set_cursor(2);
    /// assert_eq!(cursor.cur_cursor(), 2);
    /// ```
    pub fn set_cursor(&mut self, offset: usize) {
        if offset != self.offset {
            self.offset = offset;
            self.state = if offset == 0 || offset == self.len {
                GraphemeState::Break
            } else {
                GraphemeState::Unknown
            };
            // reset state derived from text around cursor
            self.cat_before = None;
            self.cat_after = None;
            self.ris_count = None;
        }
    }

    #[inline]
    /// The current offset of the cursor. Equal to the last value provided to
    /// `new()` or `set_cursor()`, or returned from `next_boundary()` or
    /// `prev_boundary()`.
    ///
    /// ```rust
    /// # use unicode_segmentation::GraphemeCursor;
    /// // Two flags (🇷🇸🇮🇴), each flag is two RIS codepoints, each RIS is 4 bytes.
    /// let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
    /// let mut cursor = GraphemeCursor::new(4, flags.len(), false);
    /// assert_eq!(cursor.cur_cursor(), 4);
    /// assert_eq!(cursor.next_boundary(flags, 0), Ok(Some(8)));
    /// assert_eq!(cursor.cur_cursor(), 8);
    /// ```
    pub fn cur_cursor(&self) -> usize {
        self.offset
    }

    /// Provide additional pre-context when it is needed to decide a boundary.
    /// The end of the chunk must coincide with the value given in the
    /// `GraphemeIncomplete::PreContext` request.
    ///
    /// ```rust
    /// # use unicode_segmentation::{GraphemeCursor, GraphemeIncomplete};
    /// let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
    /// let mut cursor = GraphemeCursor::new(8, flags.len(), false);
    /// // Not enough pre-context to decide if there's a boundary between the two flags.
    /// assert_eq!(cursor.is_boundary(&flags[8..], 8), Err(GraphemeIncomplete::PreContext(8)));
    /// // Provide one more Regional Indicator Symbol of pre-context
    /// cursor.provide_context(&flags[4..8], 4);
    /// // Still not enough context to decide.
    /// assert_eq!(cursor.is_boundary(&flags[8..], 8), Err(GraphemeIncomplete::PreContext(4)));
    /// // Provide additional requested context.
    /// cursor.provide_context(&flags[0..4], 0);
    /// // That's enough to decide (it always is when context goes to the start of the string)
    /// assert_eq!(cursor.is_boundary(&flags[8..], 8), Ok(true));
    /// ```
    pub fn provide_context(&mut self, chunk: &str, chunk_start: usize) {
        use crate::tables::grapheme as gr;
        assert!(chunk_start + chunk.len() == self.pre_context_offset.unwrap());
        self.pre_context_offset = None;
        if self.is_extended && chunk_start + chunk.len() == self.offset {
            let ch = chunk.chars().rev().next().unwrap();
            if self.grapheme_category(ch) == gr::GC_Prepend {
                self.decide(false); // GB9b
                return;
            }
        }
        match self.state {
            GraphemeState::Regional => self.handle_regional(chunk, chunk_start),
            GraphemeState::Emoji => self.handle_emoji(chunk, chunk_start),
            _ => {
                if self.cat_before.is_none() && self.offset == chunk.len() + chunk_start {
                    let ch = chunk.chars().rev().next().unwrap();
                    self.cat_before = Some(self.grapheme_category(ch));
                }
            }
        }
    }

    #[inline]
    fn decide(&mut self, is_break: bool) {
        self.state = if is_break {
            GraphemeState::Break
        } else {
            GraphemeState::NotBreak
        };
    }

    #[inline]
    fn decision(&mut self, is_break: bool) -> Result<bool, GraphemeIncomplete> {
        self.decide(is_break);
        Ok(is_break)
    }

    #[inline]
    fn is_boundary_result(&self) -> Result<bool, GraphemeIncomplete> {
        if self.state == GraphemeState::Break {
            Ok(true)
        } else if self.state == GraphemeState::NotBreak {
            Ok(false)
        } else if let Some(pre_context_offset) = self.pre_context_offset {
            Err(GraphemeIncomplete::PreContext(pre_context_offset))
        } else {
            unreachable!("inconsistent state");
        }
    }

    #[inline]
    fn handle_regional(&mut self, chunk: &str, chunk_start: usize) {
        use crate::tables::grapheme as gr;
        let mut ris_count = self.ris_count.unwrap_or(0);
        for ch in chunk.chars().rev() {
            if self.grapheme_category(ch) != gr::GC_Regional_Indicator {
                self.ris_count = Some(ris_count);
                self.decide((ris_count % 2) == 0);
                return;
            }
            ris_count += 1;
        }
        self.ris_count = Some(ris_count);
        if chunk_start == 0 {
            self.decide((ris_count % 2) == 0);
            return;
        }
        self.pre_context_offset = Some(chunk_start);
        self.state = GraphemeState::Regional;
    }

    #[inline]
    fn handle_emoji(&mut self, chunk: &str, chunk_start: usize) {
        use crate::tables::grapheme as gr;
        let mut iter = chunk.chars().rev();
        if let Some(ch) = iter.next() {
            if self.grapheme_category(ch) != gr::GC_ZWJ {
                self.decide(true);
                return;
            }
        }
        for ch in iter {
            match self.grapheme_category(ch) {
                gr::GC_Extend => (),
                gr::GC_Extended_Pictographic => {
                    self.decide(false);
                    return;
                }
                _ => {
                    self.decide(true);
                    return;
                }
            }
        }
        if chunk_start == 0 {
            self.decide(true);
            return;
        }
        self.pre_context_offset = Some(chunk_start);
        self.state = GraphemeState::Emoji;
    }

    #[inline]
    /// Determine whether the current cursor location is a grapheme cluster boundary.
    /// Only a part of the string need be supplied. If `chunk_start` is nonzero or
    /// the length of `chunk` is not equal to `len` on creation, then this method
    /// may return `GraphemeIncomplete::PreContext`. The caller should then
    /// call `provide_context` with the requested chunk, then retry calling this
    /// method.
    ///
    /// For partial chunks, if the cursor is not at the beginning or end of the
    /// string, the chunk should contain at least the codepoint following the cursor.
    /// If the string is nonempty, the chunk must be nonempty.
    ///
    /// All calls should have consistent chunk contents (ie, if a chunk provides
    /// content for a given slice, all further chunks covering that slice must have
    /// the same content for it).
    ///
    /// ```rust
    /// # use unicode_segmentation::GraphemeCursor;
    /// let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
    /// let mut cursor = GraphemeCursor::new(8, flags.len(), false);
    /// assert_eq!(cursor.is_boundary(flags, 0), Ok(true));
    /// cursor.set_cursor(12);
    /// assert_eq!(cursor.is_boundary(flags, 0), Ok(false));
    /// ```
    pub fn is_boundary(
        &mut self,
        chunk: &str,
        chunk_start: usize,
    ) -> Result<bool, GraphemeIncomplete> {
        use crate::tables::grapheme as gr;
        if self.state == GraphemeState::Break {
            return Ok(true);
        }
        if self.state == GraphemeState::NotBreak {
            return Ok(false);
        }
        if self.offset < chunk_start || self.offset >= chunk_start + chunk.len() {
            if self.offset > chunk_start + chunk.len() || self.cat_after.is_none() {
                return Err(GraphemeIncomplete::InvalidOffset);
            }
        }
        if let Some(pre_context_offset) = self.pre_context_offset {
            return Err(GraphemeIncomplete::PreContext(pre_context_offset));
        }
        let offset_in_chunk = self.offset - chunk_start;
        if self.cat_after.is_none() {
            let ch = chunk[offset_in_chunk..].chars().next().unwrap();
            self.cat_after = Some(self.grapheme_category(ch));
        }
        if self.offset == chunk_start {
            let mut need_pre_context = true;
            match self.cat_after.unwrap() {
                gr::GC_Regional_Indicator => self.state = GraphemeState::Regional,
                gr::GC_Extended_Pictographic => self.state = GraphemeState::Emoji,
                _ => need_pre_context = self.cat_before.is_none(),
            }
            if need_pre_context {
                self.pre_context_offset = Some(chunk_start);
                return Err(GraphemeIncomplete::PreContext(chunk_start));
            }
        }
        if self.cat_before.is_none() {
            let ch = chunk[..offset_in_chunk].chars().rev().next().unwrap();
            self.cat_before = Some(self.grapheme_category(ch));
        }
        match check_pair(self.cat_before.unwrap(), self.cat_after.unwrap()) {
            PairResult::NotBreak => return self.decision(false),
            PairResult::Break => return self.decision(true),
            PairResult::Extended => {
                let is_extended = self.is_extended;
                return self.decision(!is_extended);
            }
            PairResult::Regional => {
                if let Some(ris_count) = self.ris_count {
                    return self.decision((ris_count % 2) == 0);
                }
                self.handle_regional(&chunk[..offset_in_chunk], chunk_start);
                self.is_boundary_result()
            }
            PairResult::Emoji => {
                self.handle_emoji(&chunk[..offset_in_chunk], chunk_start);
                self.is_boundary_result()
            }
        }
    }

    #[inline]
    /// Find the next boundary after the current cursor position. Only a part of
    /// the string need be supplied. If the chunk is incomplete, then this
    /// method might return `GraphemeIncomplete::PreContext` or
    /// `GraphemeIncomplete::NextChunk`. In the former case, the caller should
    /// call `provide_context` with the requested chunk, then retry. In the
    /// latter case, the caller should provide the chunk following the one
    /// given, then retry.
    ///
    /// See `is_boundary` for expectations on the provided chunk.
    ///
    /// ```rust
    /// # use unicode_segmentation::GraphemeCursor;
    /// let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
    /// let mut cursor = GraphemeCursor::new(4, flags.len(), false);
    /// assert_eq!(cursor.next_boundary(flags, 0), Ok(Some(8)));
    /// assert_eq!(cursor.next_boundary(flags, 0), Ok(Some(16)));
    /// assert_eq!(cursor.next_boundary(flags, 0), Ok(None));
    /// ```
    ///
    /// And an example that uses partial strings:
    ///
    /// ```rust
    /// # use unicode_segmentation::{GraphemeCursor, GraphemeIncomplete};
    /// let s = "abcd";
    /// let mut cursor = GraphemeCursor::new(0, s.len(), false);
    /// assert_eq!(cursor.next_boundary(&s[..2], 0), Ok(Some(1)));
    /// assert_eq!(cursor.next_boundary(&s[..2], 0), Err(GraphemeIncomplete::NextChunk));
    /// assert_eq!(cursor.next_boundary(&s[2..4], 2), Ok(Some(2)));
    /// assert_eq!(cursor.next_boundary(&s[2..4], 2), Ok(Some(3)));
    /// assert_eq!(cursor.next_boundary(&s[2..4], 2), Ok(Some(4)));
    /// assert_eq!(cursor.next_boundary(&s[2..4], 2), Ok(None));
    /// ```
    pub fn next_boundary(
        &mut self,
        chunk: &str,
        chunk_start: usize,
    ) -> Result<Option<usize>, GraphemeIncomplete> {
        if self.offset == self.len {
            return Ok(None);
        }
        let mut iter = chunk[self.offset - chunk_start..].chars();
        let mut ch = iter.next().unwrap();
        loop {
            if self.resuming {
                if self.cat_after.is_none() {
                    self.cat_after = Some(self.grapheme_category(ch));
                }
            } else {
                self.offset += ch.len_utf8();
                self.state = GraphemeState::Unknown;
                self.cat_before = self.cat_after.take();
                if self.cat_before.is_none() {
                    self.cat_before = Some(self.grapheme_category(ch));
                }
                if self.cat_before.unwrap() == GraphemeCat::GC_Regional_Indicator {
                    self.ris_count = self.ris_count.map(|c| c + 1);
                } else {
                    self.ris_count = Some(0);
                }
                if let Some(next_ch) = iter.next() {
                    ch = next_ch;
                    self.cat_after = Some(self.grapheme_category(ch));
                } else if self.offset == self.len {
                    self.decide(true);
                } else {
                    self.resuming = true;
                    return Err(GraphemeIncomplete::NextChunk);
                }
            }
            self.resuming = true;
            if self.is_boundary(chunk, chunk_start)? {
                self.resuming = false;
                return Ok(Some(self.offset));
            }
            self.resuming = false;
        }
    }

    /// Find the previous boundary after the current cursor position. Only a part
    /// of the string need be supplied. If the chunk is incomplete, then this
    /// method might return `GraphemeIncomplete::PreContext` or
    /// `GraphemeIncomplete::PrevChunk`. In the former case, the caller should
    /// call `provide_context` with the requested chunk, then retry. In the
    /// latter case, the caller should provide the chunk preceding the one
    /// given, then retry.
    ///
    /// See `is_boundary` for expectations on the provided chunk.
    ///
    /// ```rust
    /// # use unicode_segmentation::GraphemeCursor;
    /// let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
    /// let mut cursor = GraphemeCursor::new(12, flags.len(), false);
    /// assert_eq!(cursor.prev_boundary(flags, 0), Ok(Some(8)));
    /// assert_eq!(cursor.prev_boundary(flags, 0), Ok(Some(0)));
    /// assert_eq!(cursor.prev_boundary(flags, 0), Ok(None));
    /// ```
    ///
    /// And an example that uses partial strings (note the exact return is not
    /// guaranteed, and may be `PrevChunk` or `PreContext` arbitrarily):
    ///
    /// ```rust
    /// # use unicode_segmentation::{GraphemeCursor, GraphemeIncomplete};
    /// let s = "abcd";
    /// let mut cursor = GraphemeCursor::new(4, s.len(), false);
    /// assert_eq!(cursor.prev_boundary(&s[2..4], 2), Ok(Some(3)));
    /// assert_eq!(cursor.prev_boundary(&s[2..4], 2), Err(GraphemeIncomplete::PrevChunk));
    /// assert_eq!(cursor.prev_boundary(&s[0..2], 0), Ok(Some(2)));
    /// assert_eq!(cursor.prev_boundary(&s[0..2], 0), Ok(Some(1)));
    /// assert_eq!(cursor.prev_boundary(&s[0..2], 0), Ok(Some(0)));
    /// assert_eq!(cursor.prev_boundary(&s[0..2], 0), Ok(None));
    /// ```
    pub fn prev_boundary(
        &mut self,
        chunk: &str,
        chunk_start: usize,
    ) -> Result<Option<usize>, GraphemeIncomplete> {
        if self.offset == 0 {
            return Ok(None);
        }
        if self.offset == chunk_start {
            return Err(GraphemeIncomplete::PrevChunk);
        }
        let mut iter = chunk[..self.offset - chunk_start].chars().rev();
        let mut ch = iter.next().unwrap();
        loop {
            if self.offset == chunk_start {
                self.resuming = true;
                return Err(GraphemeIncomplete::PrevChunk);
            }
            if self.resuming {
                self.cat_before = Some(self.grapheme_category(ch));
            } else {
                self.offset -= ch.len_utf8();
                self.cat_after = self.cat_before.take();
                self.state = GraphemeState::Unknown;
                if let Some(ris_count) = self.ris_count {
                    self.ris_count = if ris_count > 0 {
                        Some(ris_count - 1)
                    } else {
                        None
                    };
                }
                if let Some(prev_ch) = iter.next() {
                    ch = prev_ch;
                    self.cat_before = Some(self.grapheme_category(ch));
                } else if self.offset == 0 {
                    self.decide(true);
                } else {
                    self.resuming = true;
                    self.cat_after = Some(self.grapheme_category(ch));
                    return Err(GraphemeIncomplete::PrevChunk);
                }
            }
            self.resuming = true;
            if self.is_boundary(chunk, chunk_start)? {
                self.resuming = false;
                return Ok(Some(self.offset));
            }
            self.resuming = false;
        }
    }
}

#[test]
fn test_grapheme_cursor_ris_precontext() {
    let s = "\u{1f1fa}\u{1f1f8}\u{1f1fa}\u{1f1f8}\u{1f1fa}\u{1f1f8}";
    let mut c = GraphemeCursor::new(8, s.len(), true);
    assert_eq!(
        c.is_boundary(&s[4..], 4),
        Err(GraphemeIncomplete::PreContext(4))
    );
    c.provide_context(&s[..4], 0);
    assert_eq!(c.is_boundary(&s[4..], 4), Ok(true));
}

#[test]
fn test_grapheme_cursor_chunk_start_require_precontext() {
    let s = "\r\n";
    let mut c = GraphemeCursor::new(1, s.len(), true);
    assert_eq!(
        c.is_boundary(&s[1..], 1),
        Err(GraphemeIncomplete::PreContext(1))
    );
    c.provide_context(&s[..1], 0);
    assert_eq!(c.is_boundary(&s[1..], 1), Ok(false));
}

#[test]
fn test_grapheme_cursor_prev_boundary() {
    let s = "abcd";
    let mut c = GraphemeCursor::new(3, s.len(), true);
    assert_eq!(
        c.prev_boundary(&s[2..], 2),
        Err(GraphemeIncomplete::PrevChunk)
    );
    assert_eq!(c.prev_boundary(&s[..2], 0), Ok(Some(2)));
}

#[test]
fn test_grapheme_cursor_prev_boundary_chunk_start() {
    let s = "abcd";
    let mut c = GraphemeCursor::new(2, s.len(), true);
    assert_eq!(
        c.prev_boundary(&s[2..], 2),
        Err(GraphemeIncomplete::PrevChunk)
    );
    assert_eq!(c.prev_boundary(&s[..2], 0), Ok(Some(1)));
}